Frequently used material handling equipment in the storage
yard is the gantry (yard) crane (GC), shown in Fig. 2a. There are
two types of gantry cranes, rubber-tyred gantry cranes (RTGCs)
and rail-mounted gantry cranes (RMGCs). RTGCs are typically
non-automated as they have the flexibility to travel freely within
and between blocks. RTGCs are able to rotate the tires 90 to perform
orthogonal moves known as cross gantrying. However, cross
gantrying is a very slow process that takes approximately 15 minutes.
On the other hand, the movement of RMGCs is limited by its
(underground) rails.